1. Field of Invention
The present invention relates generally to systems and methods of inspecting semiconductor wafers, and relates more specifically to a semiconductor wafer inspection system, and method capable of detecting and measuring wafer defect's in which the scattering power of the defect exceeds the dynamic range of the system.
2. Description of Related Art
Systems and methods of inspecting workpieces and especially semiconductor wafers are known for detecting and measuring defects occurring on a surface of a semiconductor wafer. For example, a conventional laser-based surface scanning inspection system is typically configured to detect localized light scatters on a semiconductor wafer surface. Such localized light scatters may be indicative of one or more defects on the wafer surface that may render an integrated circuit(s) (IC) fabricated on the wafer to be inoperative. In a typical mode of operation, the conventional surface scanning inspection system sweeps a laser light beam in a predetermined direction, while the wafer being inspected rotates under the swept beam at an angle of about 90° to the predetermined sweep direction. Next, the conventional surface scanning inspection system detects a light beam reflected from the wafer surface, and samples the detected signal in both the predetermined direction of the swept beam and in the direction of rotation to obtain a two-dimensional array of data. When the light beam sweeps over a defect on the wafer surface, the data obtained by the wafer inspection system generally corresponds to the beam shape of the laser spot power at the wafer surface. This is because such wafer surface defects are generally much smaller than the spot size of the laser beam. After the conventional surface scanning inspection system has detected a defect, the system may attempt to measure the size of the defect by determining the value of the maximum scattering power of the defect, and may also determine the location of the defect on the surface of the wafer.
One drawback of the above-described conventional laser-based surface scanning inspection system is that the maximum scattering power of a detected defect may exceed the dynamic range of the system. As a result, the electronics within the wafer inspection system may saturate, thereby causing at least some of the defect size measurements performed by the system to be at a power level at which the measurements become nonlinear due to the saturation effects.
One way of addressing the effects of saturation on defect size measurements made by the conventional laser-based surface scanning inspection system is to employ a data extrapolation technique. However, such data extrapolation techniques are often difficult to perform in conventional wafer inspection systems. Alternatively, the conventional surface scanning inspection system may perform a nonlinear least squares fit of the measurements to a given Gaussian shape, which may be characterized by a number of parameters including an estimated amplitude, an estimated inverse correlation matrix, and an estimated pulse center location. However, conventional algorithms for performing such nonlinear least squares fit techniques often require a significant amount of processing time. Further, relatively small changes in the data resulting from, e.g., noise or a non-ideal signal, may lead to significantly large changes in the estimated parameters.
One methodology that can measure very large defects takes advantage of correlations between a scatter light response for a defect of unknown size, and a scatter light response for a defect of known size. For these large defects, a set-point threshold for scatter light intensity, or the equivalent voltage representation is used to contribute to defining a scatter light response area where the set-point threshold is exceeded. The defined response area is compared to response areas for known defect sizes, determined through calibration processes, and an estimated size of the measured unknown defect can be obtained. The methodology uses an empirical calibration between area and defect or particle size, and has proven to be quite robust. There is a drawback to the methodology in that it adds some difficulty because it uses a completely separate calibration process, which can be time consuming and cumbersome.
Another methodology calculates the area of a cross section of a pulse shape representation of the scatter light response at multiple non-saturated signal levels. The methodology calculates a fit line for area versus logarithm of the signal level. The fit line and non-saturated measurements are used to extrapolate to a pulse cross-section of zero area to estimate peak voltage corresponding to a peak of the pulse shape representation. The methodology also incorporates a verification of the slope of the fit line to match a slope from an expected Gaussian pulse response. This methodology has the drawback of requiring an unclipped or unsaturated signal level. If the signal is clipped, correction of areas for filter effects is imperfect and performance may be degraded. This methodology is also limited in range over which extrapolation is useful, as well as being computationally intensive.
Another technique uses an area of a scatter light response cross section at a single unsaturated signal level and then extrapolates to determine a response peak using a slope expected from a Gaussian pulse response. However, this technique also uses an unclipped signal and can be extremely sensitive to variations from an ideal Gaussian response.
It would therefore be desirable to have an improved system and method of inspecting semiconductor wafers that can measure the size and determine the location of a defect on a surface of a semiconductor wafer while avoiding the drawbacks of conventional wafer inspection systems and methods.